1. Field of the Invention
The present invention relates to an electrically erasable programmable nonvolatile semiconductor memory, and particularly, to an electrically erasable programmable nonvolatile semiconductor memory having a dual operation function.
2. Description of the Related Art
In recent years, among electrically erasable programmable nonvolatile semiconductor memories, flash memories, or FEEPROMs (flash-type electrically erasable programmable read only memories) have been highlighted because they realize large capacity at low cost. The flash memories have a read mode, a write mode, and an erase mode with respect to memory cells contained therein. Each of the modes involves a read operation to read data out of the memory cells.
The erase mode needs a longer time to complete than the read or write mode. To cope with the long time of the erase mode, some flash memories have a dual operation (simultaneous operation) function that divides a memory cell array into two blocks and carries out an erase (erase or write) operation on one of the blocks and a read or write (read) operation on the other. These;e write and erase operations are automatic write and erase operations that are started in response to an external command sequence and are completed within a semiconductor chip.
The dual operation function complicates and enlarges the flash memories. It is impossible to select the bit structure of each block when the memory cell array of the flash memories into blocks.
The details and problems of flash memories of the prior and related arts will be described in detail, later, with reference to the drawings.